Melissa Benoist as Supergirl
The trailer for Season 2, episode 21 of “Supergirl” teases Lillian Luthor (Brenda Strong) and Cat Grant’s (Calista Flockhart) return to National City. The CW

It looks like Kara (Melissa Benoist) is left with no other choice but to join forces with Lillian Luthor (Brenda Strong) in next week’s Season 2, episode 21 of “Supergirl.”

In the trailer for the episode, Lillian unexpectedly shows up in front of Kara and her friends as Rhea and her legion of Daxamites start invading Earth. Alex (Chyler Leigh), Maggie (Floriana Lima) and Guardian (Mehcad Brooks) are quick to pull out their weapons on Lillian, but it seems that the head of Cadmus isn’t there to start trouble, but to form an alliance. Later in the preview clip, Maggie and Guardian are seen listening to Lillian as she appears to lay out her plan to stop the Daxamites from dominating Earth.

Kara’s alliance with Lillian shouldn’t come as a surprise to fans because Benoist teased to Entertainment Weekly last month that the Girl of Steel might ultimately team up with one of her enemies to take down a more evil one at the end of the season. “She’s going to have to figure out how to balance it all,” Benoist said. “There might be people she has to ally with and people who will be her clear enemies, so it’s a lot of juggling.”

READ: “Supergirl” EP teases General Zod’s appearance

In addition to Lillian, Cat Grant (Calista Flockhart) also returns to National City in the episode. As seen in the trailer, Cat addresses the public saying: “Good evening, National City. I’ve been away for awhile, but I am back.”

In an interview with E! News earlier this month, executive producer Andrew Kreisberg said that Kara is going to need as much advice as possible because “she’s forced to make several very difficult decisions” in the final two episodes of the season. “Obviously, it’s always exciting to have her fight aliens and perform miracles and catch spaceships and rescue people, but she’s forced to make some of the most difficult decisions she’s ever had to make in both episodes 21 and especially 22,” Kreisberg said. “And every character, including … Cat, is there to provide her with the emotional support to make some of these decisions.” The exec producer added that Cat has a “great standoff” with Rhea in next week’s episode.

President Olivia Marsdin (Lynda Carter) also makes a comeback in episode 21. According to the synopsis for the hour, the President gives Kara an order regarding Rhea’s latest actions, which the Kryptonian grapples whether or not to obey.

“In episode 21, you have a scene with Teri Hatcher and Calista Flockhart and Melissa Benoist and Lynda Carter in a script written by Jessica Queller, directed by Millicent Shelton,” Kreisberg revealed to E! News. “It really is a woman's world on this show and we're so proud to be able to present that.”

“It’s amazing to me to be on set when you're watching Teri Hatcher, Calista Flockhart and Lynda Carter all yelling at each other. It's pretty cool,” added the exec producer.

Meanwhile, Rhea’s plan for Mon-El (Chris Wood) and Lena (Katie McGrath) remains unclear. But in the trailer for episode, the two are seen dressed in Daxamite garbs during what appears to be a ceremony. Are the two getting married? Only time will tell.

“Supergirl” Season 2, episode 21, titled “Resist,” airs on Monday, May 15 at 8 p.m. EDT on The CW. Watch the trailer below: